This invention relates to fossil-fuel fired power generation systems of the type that include at least a fossil fuel-fired steam generator and an air preheater, and more specifically, to a method of operating such fossil fuel-fired power generation systems in order to thereby optimize the operating efficiency, and particularly the low load efficiency, thereof.
In the most simplest of terms, one may define the operating efficiency of a fossil fuel-fired power generation system to be the ratio of the heat output that is derived from the fossil fuel-fired power generation system divided by the heat input of the fuel that is consumed in generating such a heat output from the fossil fuel-fired power generation system. As such, the more heat transfer which takes place within the fossil fuel-fired power generation system the lower will be the gas temperature exiting from the air preheater of the fossil fuel-fired power generation system. Moreover, the lower the gas temperature exiting from the air preheater, the higher will be the operating efficiency of the fossil fuel-fired power generation system.
Yet, it would be undesirable to lower the gas temperature exiting from the air preheater below a certain level. The temperature level to which reference is had here is the acid dewpoint temperature. To this end, if the temperature of the gas exiting from the air preheater were to be lower than the acid dewpoint temperature, a condition would thereby be created wherein the water vapor in the gas could condense out of the gas on to surfaces of the air preheater. This in turn could result in corrosion occurring particularly at the cold end, i.e., gas exit end, of the air preheater. Thus, it is desirable from the standpoint of optimizing the operating efficiency of the fossil fuel-fired power generation system that the temperature of the gas exiting from the air preheater be as low as possible, yet on the other hand from the standpoint of minimizing the possibility that the air preheater, and in particular the gas exit end thereof, not be subjected to corrosion, it is desirable that the temperature of the gas exiting from the air preheater remain above the acid dewpoint temperature.
When a fossil fuel-fired power generation system includes other components in addition to a fossil fuel-fired steam generator and an air preheater the ability to attain an optimization of the operating efficiency of the fossil fuel-fired power generation system is rendered even more difficult. An example of such another component which one finds more and more frequently being incorporated into a fossil fuel-fired power generation system is a NO.sub.x emission-reduction component. In this regard, the reduction of nitrogen oxides (NO.sub.x) emissions from stationary combustion sources, such as fossil fuel-fired power generation systems, has become a critical issue in most industrialized nations of the World. As a result, the technology associated with the control of NO.sub.x emissions from fossil fuel-fired power generation systems has matured and expanded significantly.
The NO.sub.x emissions reduction processes available for use with fossil fuel-fired power generation systems through NO.sub.x control within the fossil fuel-fired steam generator, such as by means of, for example, overfire air, gas recirculation, reduced-excess-air firing, gas mixing, low-NO.sub.x concentric tangential firing, staged combustion, fluidized-bed firing, etc.; and through post combustion NO.sub.x control effected downstream of the fossil fuel-fired steam generator primarily through the use of selective catalytic reduction (SCR) equipment, provide several alternatives for meeting strict nitrogen-oxide emission levels. Depending on the NO.sub.x emission level required, an optimum NO.sub.x reduction system may result in the integration of several of the above techniques in the overall fossil fuel-fired power generation system.
After NO.sub.x control has been-implemented within the fossil fuel-fired steam generator through the use of any one or more of the methods enumerated hereinabove, post combustion controls can result in further NO.sub.x emission reduction. With dry selective catalytic reduction equipment NO.sub.x reductions of eighty to ninety percent are claimed to be achievable.
The selective catalytic reduction process was originally developed for those applications where strict NO.sub.x emission requirements dictate the use of post-combustion NO.sub.x reduction techniques. The selective catalytic reduction process was initially applied to natural gas-fired power generation systems, then to low and high sulfur oil-fired power generation systems, and finally to coal-fired power generation systems.
The selective catalytic reduction system uses a catalyst and a reductant, e.g., ammonia gas, i.e., NH.sub.3, to dissociate NO.sub.x to nitrogen gas and water vapor. The catalytic-reactor chamber is typically located between the economizer outlet of the fossil fuel-fired steam generator and the flue-gas inlet of the air preheater of the fossil fuel-fired power generation system. This location is typical for fossil fuel-fired power generation systems with selective catalytic reduction system operating temperatures of 575.degree. F. to 750.degree. F., i.e., 300.degree. C. to 400.degree. C.
Upstream of the selective catalytic reduction chamber are the ammonia injection pipes, nozzles, and mixing grid. Through orifice openings in the ammonia injection nozzles, a diluted mixture of ammonia gas in air is dispersed into the flue-gas stream. After the mixture diffuses, it is further distributed in the gas stream by a grid of carbon steel piping in the flue-gas duct. The ammonia/flue-gas mixture then enters the reactor where the catalytic reaction is completed.
Insofar as the optimization of the operating efficiency of a fossil fuel-fired power generation system that incorporates selective catalytic reduction, i.e., an SCR, is concerned, such optimization of the operating efficiency thereof must be attained while yet ensuring that the operating temperature requirements of the SCR are satisfied. In this regard, as has been noted herein previously, in order to realize the performance desired therefrom the SCR must be located within the fossil fuel-fired power generation system such that the operating temperature to which the SCR is subjected is between 575.degree. F. and 750.degree. F. Not only must this temperature range be maintained for the SCR when the fossil fuel-fired power generation is being operated at full load, but also must be maintained when the fossil fuel-fired power generation system is being operated other than at full load. Moreover, when possible it is desirable to design for higher inlet temperatures to the SCR in order to thereby render it possible to employ lower required volumes of catalyst and concomitantly achieve potentially higher efficiency of the catalyst over its life. In addition, it is desirable to be able to operate the catalyst at higher temperatures for specific periods of time to "recover" catalyst performance efficiency which may have been lost by virtue of the fact that the catalyst, although undesirable, has nevertheless been subjected to temperatures lower than the gas temperature recommended for catalyst, i.e., SCR, operation. To thus summarize, in the case of fossil fuel-fired power generation systems that incorporate an SCR therewithin, there is a need insofar as the optimization of the operating efficiency of such a fossil fuel-fired power generation system is concerned to ensure that in addition to the considerations previously discussed herein that impact upon the ability to attain such optimization there must also be taken into consideration the temperature requirements associated with the operation of the SCR.
With further regard to optimizing the operating efficiency of a fossil fuel-fired power generation system, it is desirable that such optimization prevail over the entire load range that it is contemplated that the fossil fuel-fired power generation system will be employed. In order to maximize the flexibility of controlling the gas temperature leaving the fossil fuel-fired power generation system, a flue gas bypass around a portion of the heat transfer surface thereof can be utilized. Such a bypass will have the effect of varying the gas temperature leaving the air preheater of the fossil fuel-fired power generation system and, therefore, will also have the effect of varying the overall thermal efficiency of the fossil fuel-fired power generation system.
In addition, with regard to acid dewpoint considerations, there are many outside factors that contribute to the determination of the acid dewpoint temperature (e.g., % sulphur in the coal, % oxygen in the flue gas, and ambient air temperature). Thus, it is desirable to have the capability to be able to maintain the flexibility of varying the gas temperature to react to these changing factors.
While bypassing gas around a portion of the heat transfer surface, i.e., the economizer of the fossil fuel-fired power generation system, to maintain a minimum gas inlet temperature to the SCR while reducing load, it has been common heretofore for one to find that the temperature of the gas exiting from the air preheater rises as the load output of the fossil fuel-fired power generation system decreases. This is attributable in large part to the fact that as the load output of the fossil fuel-fired power generation system decreases, while maintaining a constant gas temperature entering the air preheater (and SCR), normally less air is made to flow through the air preheater. Accordingly, since thus there is less air to be heated within the air preheater, less heat will be transferred for this purpose from the gas flowing through the air preheater to the air flowing through the air preheater. As such, the gas is, therefore, cooled to a lesser degree as the gas flows through the air preheater thereby resulting in less of a decrease in the temperature of the gas as the gas flows through the air preheater, i.e., an increase in the temperature of the gas exiting from the air preheater as contrasted to the temperature of the gas exiting from the air preheater when the fossil fuel-fired power generation system is being operated at full load. To thus summarize, just because the optimization of the operating efficiency of the fossil fuel-fired power generation system is attained when the fossil fuel-fired power generation system is being operated at full load does not ensure that such optimization of the operating efficiency of the fossil fuel-fired power-generation system will likewise be attained when the fossil fuel-fired power generation system is being operated other than at full load.
As noted herein previously it has been known heretofore to provide fossil fuel-fired power generation systems having NO.sub.x reduction equipment incorporated therewithin. In this regard, by way of exemplification and not limitation, one such fossil fuel-fired power generation system is that which forms the subject matter of U.S. Pat. No. 4,160,009, which issued on Jul. 3, 1979 and which is entitled "Boiler Apparatus Containing Denitrator." In accordance with the teachings of U.S. Pat. No. 4,160,009 there is provided in a boiler apparatus having a furnace and a plurality of heat exchangers disposed in a combustion gas channel between the furnace and boiler apparatus exits, the improvement comprising a denitrator having a catalyst disposed in the combustion gas channel downstream of at least one of the heat exchangers, a bypass duct for the combustion gas channel connecting a first region thereof in which the denitrator is disposed with a second region upstream of the first region, control valve means disposed in the duct, and a temperature detector disposed in the first region and connected to the control valve means so as to control the opening and the closing of the valve means in response to the temperature detected in the first region by the detector.
By way of exemplification and not limitation, a second such fossil fuel-fired power generation system is that which forms the subject matter of U.S. Pat. No. 4,220,633, which issued on Sep. 2, 1980 and which is entitled "Filter House and Method for Simultaneously Removing NO.sub.x and Particulate Matter from a Gas Stream." In accordance with the teachings of U.S. Pat. No. 4,220,633, there is provided a vapor generator and a filter house, the latter being disposed between the vapor generator and an air preheater. An ammonia storage tank is positioned to introduce ammonia via an ammonia distribution grid into the flue gas inlet conduit through which flue gas is transported from the vapor generator to the filter house. The filter house is designed to be operative for removing or cleansing NO.sub.x emissions from the flue gas stream transported thereto during the passage thereof through the filter house while simultaneously filtering out entrained particulate matter from the same flue gas stream.
By way of exemplification and not limitation, a third such fossil fuel-fired power generation system is that which forms the subject matter of U.S. Pat. No. 5,047,220, which issued on Sep. 10, 1991 and which is entitled "Catalytic Denitrification Control Process and System for Combustion Flue Gases." In accordance with the teachings of U.S. Pat. No. 5,047,220, there is provided a control process and system for ammonia addition to combustion flue gas streams containing excessive nitrogen oxides (NO.sub.x) upstream of a catalytic denitrification unit. The control system includes a source of hot combustion gases such as that produced from a boiler or a gas turbine power plant, nozzle means for injecting ammonia into the flue gas stream, a catalytic denitrification unit provided in the flue gas stream downstream of the ammonia injection nozzle means, and an exhaust conduit or stack leading to the atmosphere and including a gas sampling and NO.sub.x analyzer device.
By way of exemplification and not limitation, a fourth such fossil fuel-fired power generation system is that which forms the subject matter of U.S. Pat. No. 5,151,256, which issued on Sep. 29, 1992 and which is entitled "Coal Combustion Apparatus Provided with a Denitration." In accordance with the teachings of U.S. Pat. No. 5,151,256 there is provided a coal combustion apparatus comprising a combustion furnace, a denitration means for removing nitrogen oxides contained in an exhaust gas from the combustion furnace by reducing the oxides with ammonia as a reducing agent, a means for collecting ashes contained in the exhaust gas having left the denitration means and a means for recycling the collected ashes into the combustion furnace, which apparatus is provided with an oxygen concentration meter in the flow path of the exhaust gas between the combustion furnace and the denitration means and also provided with an oxygen concentration-controlling means relative to air fed inside the flow path of the exhaust gas from the combustion furnace to the denitration means so as to control the oxygen concentration detected by the oxygen concentration meter to a definite value or higher.
Although fossil fuel-fired power generation systems constructed in accordance with the teachings of the four issued U.S. patents to which reference has been made hereinbefore have been demonstrated to be operative for the purpose for which they have been designed, there has nevertheless been evidenced in the prior art a need for the operation of such a fossil fuel-fired power generation system to be improved. More specifically, a need has been evidenced in the prior art for a new and improved method of operating such a fossil fuel-fired power generation system in order to thereby optimize the operating thermal efficiency thereof. To this end, there has been evidenced in the prior art a need for such a new and improved method of operating such a fossil fuel-fired power generation system in order to thereby optimize the operating thermal efficiency thereof, which would be advantageously characterized in a number of respects.
By way of exemplification and not limitation in this regard, one such advantageous characteristic that such a new and improved method of operating a fossil fuel-fired power generation system in order to optimize the operating efficiency thereof would desirably embody is that it would be employable with fossil fuel-fired power generation systems that incorporate an SCR as well as with fossil fuel-fired power generation systems that do not incorporate an SCR. A, second such advantageous characteristic that such a new and improved method of operating a fossil fuel-fired power generation system in order to thereby optimize the operating efficiency thereof would desirably embody is the capability to attain such optimization of the operating efficiency thereof both when the fossil fuel-fired power generation system is being operated at full load and when the fossil fuel-fired power generation system is being operated at other than full load. A third such advantageous characteristic that such a new and improved method of operating a fossil fuel-fired power generation system in order to thereby optimize the operating efficiency thereof would desirably embody is the capability to attain such optimization of the operating efficiency thereof while at the same time ensuring that the temperature of the gas exiting from the air preheater remains as desired relative to the dewpoint temperature. A fourth such advantageous characteristic that such a new and improved method of operating a fossil fuel-fired power generation system in order to thereby optimize the operating efficiency thereof would desirably embody is the capability to attain such optimization of the operating efficiency thereof while at the same time ensuring that when the fossil fuel-fired power generation system incorporates an SCR that the range of operating temperatures required for the operation of the SCR is maintained.
To thus summarize, a need has been evidenced in the prior art for such a new and improved method of operating a fossil fuel-fired power generation system in order to thereby optimize the operating thermal efficiency thereof which enables optimum control of the air preheater exit gas temperature to be had such that 1) it is thereby possible to alter the air preheater exit gas temperature profile over load in order to thus provide optimum combinations of fossil fuel-fired steam generator efficiency, 2) it is thereby possible to provide protection against corrosion at the air inlet end, i.e., the cold end, of the air preheater due to too low air preheater exit gas temperature, and 3) it is thereby possible to control gas temperature into and around the SCR, when the fossil fuel-fired power generation system is equipped therewith, in order to thus provide for the optimization of the life of the NO.sub.x catalyst as well as to provide for the optimization of the injection of the ammonia associated with the use of the SCR.
It is, therefore, an object of the present invention to provide a new and improved method of operating a fossil fuel-fired power generation system in order to thereby be able to attain optimization of the operating efficiency of such a fossil fuel-fired power generation system.
It is a further object of the present invention to provide such a method of operating a fossil fuel-fired power generation system in order to thereby be able to attain optimization of the operating efficiency of such a fossil fuel-fired power generation system when such a fossil fuel-fired power generation system does not incorporate selective catalytic reduction equipment, i.e., an SCR.
It is another object of the present invention to provide such a method of operating a fossil fuel-fired power generation system in order to thereby be able to attain optimization of the operating efficiency of such a fossil fuel-fired power generation system when such a fossil fuel-fired power generation system incorporates selective catalytic reduction equipment, i.e., an SCR.
Another object of the present invention is to provide such a method of operating a fossil fuel-fired power generation system in order to thereby be able to attain optimization of the operating efficiency of such a fossil fuel-fired power generation system while yet at the same time ensuring that the air preheater, and particularly the gas exit end thereof, of such a fossil fuel-fired power generation system is protected against corrosion caused by the gas exit temperature falling below the acid dewpoint temperature.
A still another object of the present invention is to provide such a method of operating a fossil fuel-fired power generation system in order to thereby be able to attain optimization of the operating efficiency of such a fossil fuel-fired power generation system when such a fossil fuel-fired power generation system is being operated at full load.
A further object of the present invention is to provide such a method of operating a fossil fuel-fired power generation system in order to thereby be able to attain optimization of the operating efficiency of such a fossil fuel-fired power generation system when such a fossil fuel-fired power generation system is bring operated at other than full load.
A still further object of the present invention is to provide such a method of operating a fossil fuel-fired power generation system in order to thereby be able to attain optimization of the operating efficiency of such a fossil fuel-fired power generation system while yet at the same time ensuring that the range of temperatures required for the selective catalytic reduction equipment, i.e., the SCR, is still satisfied.
Yet an object of the present invention is to provide such a method of operating a fossil fuel-fired power generation system in order to thereby be able to attain optimization of the operating efficiency of such a fossil fuel-fired power generation system when such a fossil fuel-fired power generation system is being employed in a new application.
Yet a further object of the present invention is to provide such a method of operating a fossil fuel-fired power generation system in order to thereby be able to attain optimization of the operating efficiency of such a fossil fuel-fired power generation system when such a fossil fuel-fired power generation system is employed in a retrofit application.
Yet another object of the present invention is to provide such a method of operating a fossil fuel-fired power generation system in order to thereby be able to attain optimization of the operating efficiency of such a fossil fuel-fired power generation system, which is relatively easy to install and operate, while yet being relatively inexpensive to provide.